Since you're still working on it, have you considered this edge-case with floats: if a float is added long after the last breakpoint, and the current line grows long enough that it (with the float) reaches the right edge, do you take that breakpoint (and thus the float that came after moves to the next line due to the rule that a float cannot be put higher than its occurrence in the flow), or "defer" the float to the next line, freeing more space on the current line? All the browsers I've tested recently seem to do the former, but if you read the CSS rules for floats carefully, the rule that says floats are to be placed as high as possible suggests that deferring the current float, and thus possibly allowing more content on the current line, may result in future floats being higher than if the current float was not deferred, despite the current float ending up at the same height.
